1. Technical Field of the Invention
The present invention relates to the field of switching power supplies, and in particular, to a switching voltage regulator module.
2. Description of the Related Art
Advances in integrated circuit (IC) technology often relate to the ever decreasing operating voltages required to operate such circuits. A lower operating voltage may translate into lower costs due to decreases in circuit size and power consumption. Present demands for faster and more efficient data processing have prompted a significant development effort in the area of low-voltage integrated circuits. Currently, low-voltage integrated circuits operating in the three-volt range (e.g., 3.3 V ICs) are highly desirable. The three-volt ICs are gradually replacing the standard five-volt ICs due to their higher speed and higher integration densities.
Moreover, the three-volt ICs consume less power than the traditional five-volt ICs. Thus, in battery operated devices, such as portable telephones and lap-top computers, low-voltage integrated circuits allows the devices to operate proportionally longer than devices requiring higher voltage for operation.
However, the 3.3 V IC represents only a transition to ICs with even lower operating voltages that will not only further improve speed and reduce power consumption, but will also allow direct, single-cell battery consumption. It is expected that the next generation of data processing ICs will be operable at voltages in the 1-3 V range. At the same time, since more devices are integrated on a single processor chip and the processors operate at higher frequencies, microprocessors require aggressive power management. Compared with current processors, which require a current draw around 13 amps, future generation processors will require a current draw in the range of 30-50 amps. The load range may reach 1:100.
Further, as the speed of the ICs increase, they are becoming more dynamic loads to their power supplies. Next generation microprocessors are expected to exhibit current slew rates of 5 A/ns. Moreover, the output voltage regulation becomes much tighter (e.g., from 5% to 2%). Voltage regulator modules (VRMs) which feed the microprocessors have to have high efficiency, fast transient response and high power density. These requirements pose serious design challenges.
FIG. 1 is a schematic block diagram of a prior art synchronized buck converter 100. The circuit 100 is typically used as a VRM to meet the requirements of high efficiency, fast transient response and high power density. In operation, switches S1 and S2 turn on and off in complementary fashion. The voltage gain of the buck converter circuit 100 can be described by:
D=Vo/Vinxe2x80x83xe2x80x83(1)
where D is the duty ratio of switch S1.
As is well known in the art, the buck converter has a high efficiency and good transient response at around a duty cycle of 0.5. For a 5V input voltage and a 2V output, the duty cycle is 0.4, which is an acceptable duty cycle ratio for achieving high efficiency.
Since future VRMs will be required to provide more power to the microprocessors, the power switch must be able to deal with higher currents, which decreases efficiency. However, in accordance with the power equation, the increased power required by future microprocessors may be achieved by raising the input voltage instead, which allows the input current to be decreased, thereby reducing conduction losses. As such, it is preferable that VRMs have a 12V or higher input voltage. For example, the input voltage can be as high as 19V for notebook computers. According to equation (1), the duty cycle for a conventional synchronized buck converter is as small as 0.1 with a 12V input and a 1.2V output. A drawback of a duty cycle on the order of 0.1 is that the circuit exhibits poor performance in terms of efficiency, voltage regulation and transient response.
A schematic of another conventional buck converter circuit 200 is illustrated in FIG. 2. This buck converter circuit 200 is well known in the art as a tapped inductor synchronized buck converter. The tapped inductor synchronized buck converter circuit 200 operates from an unregulated supply voltage VIN and provides a regulated DC output voltage V0 at terminal 111 (e.g., 2 volts) for driving load RL which, for example, may be a microprocessor, portable or laptop computer or other battery-operated system. Circuit 200 includes power switches S1 and S2, such as a power metal oxide semiconductor field effect transistors (MOSFETS), acting in complementary fashion. Circuit 200 further includes leakage inductor Lk, coupled windings N1 and N2, and filter capacitor Co.
As is made clear below, those of ordinary skill will recognize that inductor Lk is not a separate component, but represents the leakage inductance of winding N1. Windinns N1 and N2 are coupled magnetically, and connected electrically at the tap or common junction to which the second switch is connected.
FIG. 3 illustrates various waveforms associated with circuit 200. The operation of circuit 200 will be described with reference to certain of the waveforms of FIG. 3. When switch S1 turns on during the time interval t1 to t2 (see FIG. 3a), a voltage difference, Vinxe2x88x92Vo is applied to the coupled inductor windings N1 and N2. The switching current in switch S1 linearly increases (See FIG. 3d) and the voltage across switch S2 is the input voltage (see FIG. 3f). The circuit delivers power to the output. At time t2, switch S1 turns off and switch S2 turns on (see FIG. 3b). The energy stored in winding N1 is transferred to winding N2, and the winding current is2 flows through S2 and linearly decreases (see FIG. 3c). The voltage gain of circuit 200 can be written as:
Vo/Vin=D/[1+(N1/N2)*(1xe2x88x92D)]xe2x80x83xe2x80x83(2)
where D is the duty ratio of switch S1. From equation (2) it can be seen that a duty cycle on the order of 0.5 can be realized to achieve high efficiency by properly choosing the turns ratio of the coupled inductors.
One disadvantage of circuit 200 is that a high voltage spike occurs across switch S1 when S1 turns off (e.g., at time t2, See FIG. 3e) because the leakage energy of winding N1 cannot be transferred to winding N2. The leakage energy in Lk charges the output capacitance (not shown) of S1 through conducting switch S2 which causes a high voltage stress across S1. As a result, a high voltage rated MOSFET switch must be used in the circuit 200 which significantly increases the power loss and reduces the efficiency.
It would be desirable to provide a circuit configuration which avoids the necessity of using a high voltage rated MOSFET switch and which recycles the leakage energy of the coupled inductor to further improve circuit efficiency.
It is, therefore, a primary object of the present invention to provide a circuit so that a low-voltage rated power switch can be used to improve circuit efficiency.
It is another object of the present invention to provide a circuit which recycles the leakage energy of the coupled inductor to further improve circuit efficiency.
It is yet another object of the present invention to provide a circuit which uses as few components as necessary.
In accordance with an embodiment of the present invention, there is provided an active clamp step-down converter circuit with a power switch voltage clamping function including a first switch connected in series with an unregulated DC input source, a second switch coupled at one junction at a midpoint of a coupled winding including a first winding and a second winding, a leakage inductance Lk associated with one winding of the coupled winding, a shottky diode connected in parallel with the second switch and an active clamp circuit including a clamping capacitor and a third switch, connected in series. The clamp circuit is connected in parallel with the leakage inductance and the first winding. The converter circuit further includes a filter capacitor connected to one terminal of the second winding and connected in parallel with a load.
The clamping capacitor clamps the voltage across the first switch during the period in which the first switch is off. The clamped voltage across the first switch is the sum of the input voltage and clamping capacitor voltage.
A main advantage provided by the circuit of the present invention is the prevention or substantial elimination of voltage spikes which would otherwise occur at each switch transition to the OFF state. Voltage spikes are eliminated by incorporating the active clamp circuit connected in parallel with the first winding.
A further advantage of the circuit of the present invention is that by recovering the leakage energy in each switching cycle, as opposed to dissipating it in accordance with prior art approaches, the overall circuit efficiency (i.e., power out/power in) is enhanced. An additional advantage of capturing the leakage current is that the voltage rating of the first switch is significantly reduced thereby reducing its cost.
A still further advantage of the circuit of the present invention is that the circuit is optimized to operate with a duty cycle of around 0.5 which improves the dynamic response and system efficiency. By operating with a nominal duty cycle of around 0.5, the circuit is responsive to changing load conditions. That is, when the load changes from a nominal to a heavy load, the duty cycle must be raised from 0.5 to a value close to 1 to insure that the output voltage variation remains within specification. Similarly, when the load changes from a nominal load to a light load, the duty cycle must be lowered from 0.5 to a value close to zero to insure that the output voltage variation remains within specification. The required changes in the duty cycle are most easily effected with a circuit that operates according to the present invention.